Method checkRexmitTimer

srtcore/core.cpp:12089–12165

}

void srt::CUDT::checkRexmitTimer(const steady_clock::time_point& currtime)
{
    // Check if HSv4 should be retransmitted, and if KM_REQ should be resent if the side is INITIATOR.
    checkSndTimers();

    // There are two algorithms of blind packet retransmission: LATEREXMIT and FASTREXMIT.
    //
    // LATEREXMIT is only used with FileCC.
    // The RTO is triggered when some time has passed since the last ACK from
    // the receiver, while there is still some unacknowledged data in the sender's buffer,
    // and the loss list is empty at the moment of RTO (nothing to retransmit yet).
    //
    // FASTREXMIT is only used with LiveCC.
    // The RTO is triggered if the receiver is not configured to send periodic NAK reports,
    // when some time has passed since the last ACK from the receiver,
    // while there is still some unacknowledged data in the sender's buffer.
    //
    // In case the above conditions are met, the unacknowledged packets
    // in the sender's buffer will be added to the SND loss list and retransmitted.
    //

    {
        ScopedLock ack_lock(m_RecvAckLock);
        const uint64_t rtt_syn = (m_iSRTT + 4 * m_iRTTVar + 2 * COMM_SYN_INTERVAL_US);
        const uint64_t exp_int_us = (m_iReXmitCount * rtt_syn + COMM_SYN_INTERVAL_US);

        if (currtime <= (m_tsLastRspAckTime + microseconds_from(exp_int_us)))
            return;
    }

    // If there is no unacknowledged data in the sending buffer,
    // then there is nothing to retransmit.
    if (m_pSndBuffer->getCurrBufSize() <= 0)
        return;

    const bool is_laterexmit = m_CongCtl->rexmitMethod() == SrtCongestion::SRM_LATEREXMIT; // FileCC
    const bool is_fastrexmit = m_CongCtl->rexmitMethod() == SrtCongestion::SRM_FASTREXMIT; // LiveCC

    // If the receiver will send periodic NAK reports, then FASTREXMIT (live) is inactive.
    // TODO: Probably some method of "blind rexmit" MUST BE DONE, when TLPKTDROP is off.
    if (is_fastrexmit && m_bPeerNakReport)
        return;

    // Schedule a retransmission IF:
    // - there are packets in flight (getFlightSpan() > 0);
    // - in case of LATEREXMIT (File Mode): the sender loss list is empty
    //   (the receiver didn't send any LOSSREPORT, or LOSSREPORT was lost on track).
    // - in case of FASTREXMIT (Live Mode): the RTO (rtt_syn) was triggered, therefore
    //   schedule unacknowledged packets for retransmission regardless of the loss list emptiness.
    if (getFlightSpan() > 0 && (!is_laterexmit || m_pSndLossList->getLossLength() == 0))
    {
        // Sender: Insert all the packets sent after last received acknowledgement into the sender loss list.
        ScopedLock acklock(m_RecvAckLock); // Protect packet retransmission
        // Resend all unacknowledged packets on timeout, but only if there is no packet in the loss list
        const int32_t csn = m_iSndCurrSeqNo;
        const int     num = m_pSndLossList->insert(m_iSndLastAck, csn);
        if (num > 0)
        {